A passive optical network has a point to multipoint architecture in which a single optical fibre splits to serve multiple end users. A passive optical network comprises an Optical Line Termination node (OLT) contained in the telephone exchange or central office or street side cabinet, for example, at least one optical splitter and an Optical Network Terminal node (ONT) at the end user's premises. Optical fibres connect the optical line termination node to the optical network terminal nodes. Downstream signals are broadcast to all of the optical network terminal nodes that are connected to the optical line termination node via the passive optical network. Upstream signals from the optical network terminal nodes share the medium and are sent using a Time Division Multiple Access communication scheme (TDMA).
A passive optical network provides a flexible platform for providing optical communication capabilities to multiple premises over a geographical area. Thus, the various optical network terminal nodes can be located at different distances from the optical line termination node. This inherently leads to large differences in the attenuation of signals received by the optical line termination node from the different optical network terminal nodes. The optical receiver of the optical line termination node is therefore required to handle a large range of optical input powers, where the maximum difference between a weak and a strong signal typically could be in the range of 15 to 25 dB.
Transmission from the different optical network terminal nodes is made in fairly short bursts, known as burst mode transmission, which makes it important that the optical receiver adapts to the input power variations quickly.
Optical receivers are known that include automatic gain control, which automatically alters the gain of the optical receiver on determining the signal strength. Automatic gain control circuitry has a reaction time which reflects the time it takes for the receiver to respond and settle to a change in input signal strength. There is typically a trade off between a quick reaction time and a large dynamic range that the receiver can adjust over. The dynamic range or burst-to-burst power difference is the maximum difference in the power of successive signal bursts that the automatic gain control circuitry can adjust over in a particular reaction time. If a relatively long time is allowed for the receiver to adjust and settle, the automatic gain control of the receiver can be configured to be effective over a wide dynamic range. However, this configuration leads to inefficient use of the upstream channel of the passive optical network. Alternatively, it may be easier for a receiver to achieve a short reaction time if the dynamic range is constrained to be narrow. However, this limits the range of signal strengths that the receiver can reliably operate over.
A burst mode optical receiver is disclosed in EP 1 791 275 which uses received signal strength indication information from a previous data stream of an optical network unit to adjust an optical receiver to optimize reception of subsequent incoming data streams of the optical network unit based upon received signal strength indication information received from the previous data stream.
A system in which the receiver is reset between bursts and then autonomously adjusts itself to one of two amplification ranges is disclosed in S. Nishihara et al., “A burst-mode 3R receiver for 10-Gbit/s PON systems with high sensitivity, wide dynamic range, and fast response”, Journal of Lightwave Technology, vol. 26, issue 1, January 2008.